The invention comprises chlorinating sulfide ore concentrates to convert the metal sulfides therein to chlorides from which the metals are subsequently recovered in accordance with a flow sheet to be described. The invention includes the flow sheet irrespective of the chlorination procedure, and the combination of the flow sheet with the chlorination step. Lead and silver are the principal metals which can be economically recovered from the ores. A further feature of the invention is the use of dry chlorination procedure which is particularly effective on the tetrahedrite-tennantite series of sulfide ores.
Conversion of metallic sulfides into chlorides in metal recovery processes is not broadly new. Aqueous chlorination of metal sulfide concentrates, with ferric chloride and chlorine gas in a sodium chloride or calcium chloride solution has been performed. U.S. Pat. No. 1,736,659 to Mitchell discloses a process exemplifying this mode of recovery of metal values from sulfide ores using a wet chlorination process. The process of this patent does not include recycle of leach solution or sodium chloride solution, includes a roasting step with consequent air pollution, includes removal of chloride from the system in the lead chloride, removal of iron from the system as the hydroxide rather than carbonate, and differs in other aspects from the flow sheet of this invention.
Another publication of interest is the article entitled "The Dry Chlorination of Complex Ores" by Ionidas in Mining and Scientific Press, Volume 112, May 27, 1916. This article discloses partially dry chlorinating concentrates of metal sulfides, including lead, zinc and silver sulfides, with chlorine gas with final chlorination being accomplished in a roasting step in the presence of air in which the ferric chloride formed in the chlorination step is decomposed to produce chlorine which completes the chlorination of the metal sulfides. The process is directed chiefly to the production and electrolysis of zinc chloride and is not a pollution-free process as sulfur dioxide is produced in the roasting step and released to the atmosphere. The procedure for recovering metal from the chlorides in this process lacks the features which the process of the Mitchell patent lacks as outlined above and differs in other respects from the flow sheet of the present invention.
It has been found that when the chlorination product of this invention is treated with sodium chloride to solubilize the metal chlorides, an undesirable build-up of impurities, particularly zinc chloride, in the brine leach solution occurs which adversely affects the ability of the solution after a period of time to solubilize silver and lead chlorides from the chlorinated ore product. The present process provides a means for overcoming this problem and obtaining high recoveries of silver and lead.
The tetrahedrite-tennantite polymorphic series of metal sulfide minerals is discussed at page 181 of Dana's "Manual of Mineralogy," 15th Edition, published by John Wiley and Sons, New York, New York. The formula for tetrahedrite is given as (Cu,Fe,Zn,Ag).sub.12 Sb.sub.4 S.sub.13. Arsenic may take the place of antimony in the pure arsenic end member, tennantite.
These minerals are extremely refractory to chemical leaching. As an illustration of the difficulty of leaching them, it is disclosed at page 72 of the book entitled The Chemistry of Hydrometallurgical Processes by Alfred Richard Burkin that tetrahedrite was leached with concentrated sodium sulfide solution. This is an extreme procedure.
The minerals are known to occur with other minerals such as, galena, and the amount of lead in the combined minerals has always contributed to the economic feasibility of processing this ore to recover principally lead and silver. The present high price of silver makes it economically attractive to recover the silver from the tetrahedrite-tennantite minerals irrespective of whether other minerals are associated with them. It is known that in some galena-tetrahedrite ore, some silver sulfide is in the galena while the remainder of the silver is locked in the lattice of the tetrahedrite crystals from which it is very difficult to release so that it is available for conversion to the soluble chloride from which the silver is readily recoverable by cementation or other conventional means.
It has been conventional to recover lead and silver from galena-tetrahedrite ores by pyrometallurgical processes with conversion of the sulfur present to sulfur dioxide which was released to the atmosphere with its polluting effect. In view of comparatively recent restrictions on the permissible sulfur dioxide content of the atmosphere there is a demand for non-pollution processes for recovering silver and other metals from the tetrahedrite-tennantite series of minerals alone or in combination with other minerals.
The difficulty of breaking down the minerals to make the sulfide components thereof available for chlorination is not solved by the use of conventional processes used on ores other than tetrahedrite-tennantite minerals of somewhat similar composition. For example, U.S. Pat. No. 1,736,659, mentioned above, discloses a process for recovering metals from sulfide ores including the metals lead and silver in which the metal sulfides are converted to chlorides by wet chlorination with calcium chloride solution. As the test results hereinafter presented show, wet chlorination is not effective to convert the metals of the tetrahedrite-tennantite series to chlorides without use of an excessive amount of chlorine, excessive process time and without reduced yields of metals.
A dry chlorination technique for ores containing lead, zinc and silver is disclosed in the article mentioned above entitled "The Dry Chlorination of Complex Ores" by C. A. Ionidas, dated May 27, 1916, published in Volume 112 of Mining and Scientific Press. The process was developed for processing complex sulfide ores containing zinc which could not be treated profitably by other means and is directed essentially to the recovery of zinc by fused bath electrolysis, "the most vital step in the whole process... ." The process is used on ores having a high content of iron so that only a partial chlorination is performed followed by a roasting step "in the presence of air" to regenerate chlorine consumed in the initial formation of ferric chloride. The present dry chlorination procedure cannot be conducted in the presence of oxygen, either contained in air or otherwise if a pollution-free process is desired, because the sulfur released is immediately oxidized to sulfur dioxide in the presence of oxygen. The article discloses a process performed on an ore not containing arsenic or antimony differing widely from tetrahedrite-tennantite ores. The statement on page 787 postulating that the process is effective on ores containing arsenic and antimony obviously does not include ores of the tetrahedrite-tennantite series. Certainly, a pollution-free process was not contemplated in view of the roasting step performed in the presence of air.